1. Field of the Invention
The invention relates generally to the field of ultrasound diagnostic probes and more specifically to the field of invasive ultrasound diagnostic probes.
2. Description of Related Art
FIG. 1 shows the basic construction of a previously known ultrasound diagnostic probe. The imaging probe 20 consists of a catheter 22, a piezoelectric transducer 24 (i.e., a transducer having a material that electrically polarizes when mechanically strained and that mechanically strains when electrically polarized) at the distal end 38 of the catheter (i.e., the end of the catheter that goes into the body), electric wires 26 that connect piezoelectric transducer 24 to external circuitry at the proximal end (i.e., the end that stays outside the body), an acoustic reflector 28 (i.e., a mirror) at the distal end, a rotating drive shaft 34 coupled to a small motor/shaft encoder at the proximal end, and a plastic radome 30 (i.e., an acoustic window that has the same acoustic impedance as a fluid) filled with a liquid 32 that fits over piezoelectric transducer 24 and acoustic reflector 28.
Piezoelectric transducer 24 is stationary and when excited by an external source, it produces an acoustic signal 36 that travels through the liquid in radome 30 and strikes acoustic reflector 28. The surface of acoustic reflector 28 resides at an angle of 45.degree. from acoustic signal 36 and it reflects acoustic signal 36 at an angle of 90.degree. from its original path. The reflected acoustic signal 36 travels through liquid 32 in radome 30 and propagates through the blood until it encounters the arterial wall.
Depending on the penetration into the arterial wall, several echoes return to piezoelectric transducer 24 by retracing essentially the same path. Piezoelectric transducer 24 converts these echoes into corresponding electrical pulses and wires 26 carry these electrical pulses to electrical circuitry located at the proximal end. Since acoustic reflector 28 continuously rotates, acoustic signal 36 continuously rotates. Echoes from each angular position are collected, processed and displayed on a CRT screen.
Previously known imaging probes have variations of the configuration shown in FIG. 1. In one alternate configuration, the acoustic reflector 28 and piezoelectric transducer 24 exchange places and in another alternate configuration, the reflector is omitted and the transducer is attached directly to the rotating shaft. These and other configurations have the following in common: all use a piezoelectric transducer at the distal end of the catheter, which goes inside the body, and generates a single frequency signal.
Placing the piezoelectric transducer at the distal end of the catheter that goes inside the body has numerous disadvantages. The piezoelectric transducer may emit leakage currents inside the body that can induce fibrillation when the probe images a coronary artery. Wires 26 that connect the piezoelectric transducer to external circuitry inherently act as antennas and they receive radio frequency (RF) interference present in a catheterization laboratory. This RF interference appears as noise in the electrical signals that travel to and from the piezoelectric transducer and increases the risk of fibrillation induced by the electrical signals.
Another disadvantage of placing the piezoelectric transducer at the distal end of the catheter is that after one use the piezoelectric transducer must be discarded along with the catheter to prevent the transmission of disease. This is burdensome because the piezoelectric transducers are difficult and expensive to make. It also discourages use of the most desirable transducers because they usually are more expensive. Generally, increasing the frequency of the acoustic waves improves the resolution capability of the transducer, but it also increases the expense because the output frequency of piezoelectric transducers depends upon their thickness (i.e., a 40 MHz piezoelectric transducer would have a thickness of approximately 0.05 mm) and the thinner the transducer, the more expensive the transducer.
Another disadvantage of placing the piezoelectric transducer at the distal end is that the piezoelectric transducer can only produce an acoustic signal of a single frequency. The user cannot adjust the piezoelectric transducer to obtain an acoustic signal of another frequency that will give a more desirable resolution or that illuminates a particular region of interest.
Parent patent application having Ser. No. 07/918,298, entitled Intracavity Ultrasound Diagnostic Probe Using Fiber Acoustic Waveguides, and incorporated by reference above, describes an ultrasound diagnostic probe that is a cladded-core acoustic waveguide. An acoustic waveguide typically has a central core and an outer cladding that surrounds the core. The purpose of the cladding is to confine the acoustic signals within the central core. When the materials for the cladding and the core meet the specifications described in patent application Ser. No. 07/918,298, the fiber guides the acoustic signals because they bounce off the cladding and stay inside the core.
FIG. 2 shows a previously known piezoelectric transducer 40 that is bonded to the flat end of acoustic waveguide 42 so that an acoustic signal 44 is directly coupled into acoustic waveguide 42. One problem with this configuration is that acoustic waveguides 42 are very small, approximately 0.5 mm and it is very difficult to glue a transducer to them. Another problem with this configuration is that the size of piezoelectric transducer 40 scales inversely with frequency. For example, a piezoelectric transducer 40 that produces a 40 MHz signal has a thickness around 0.05 mm (2 mils). Thus, the fabrication of this device on a production basis presents many difficulties.
FIG. 3 shows an apparatus disclosed in C. K. Jen, Ahmad Safoai-Jazi, Gerald Farnel "Leaky Modes in Weakly Guiding Fiber Acoustic Waveguides" IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. UFFC-33, No. 6, Nov. 1986, pg. 634 and U.S. Pat. No. 4,743,870 by Jen et al. This device generates a single frequency acoustic signal created by a piezoelectric transducer 50 and couples it to an acoustic waveguide 56 through a buffer rod 52 and a liquid coupling medium 54. When the acoustic signal strikes dimple 57 on buffer rod 52, this spherically curved surface focuses the energy of the acoustic wave very close to an end 58 of acoustic waveguide 56 and the acoustic signal enters acoustic waveguide 56. This arrangement has several coupling interfaces and higher coupling losses can be expected. The relatively high attenuation in blood and the proximal part of the arterial wall (i.e., the inner layers of the artery) cause the returning echoes from an intravascular imaging catheter to be small to begin with and any additional coupling losses will reduce the dynamic range of the imaging system. Another problem with this arrangement is that the frequency of the acoustic wave cannot be altered.
It is desirable to efficiently couple acoustic signals to a cladded-core acoustic waveguide and to change the frequency of the probing acoustic signal without replacing the catheter and the transducer. For example, when using a catheter tip doppler probe (i.e., a transducer attached at the tip of the catheter), it is desirable to change the frequency of the acoustic signal depending on the blood flow velocity. When using a cladded-core acoustic waveguide to image an arterial wall, it is necessary to have different frequencies to see different layers on the inside of the blood vessel, to resolve the medial layer, to differentiate between the different layers that form an occlusion such as thrombus (i.e., the top layer of plaque), soft plaque (i.e., the bottom layer of plaque), and probably to differentiate between stable and unstable plaque.